I’m Ukrainian Greek Catholic and quite frankly, the denial of the Immaculate Conception by the Orthodox really irks me.
St. Irenaeus, one of the early Catholic apologists, wrote: “Death through Eve, life through Mary.” Christ is the New Adam, Mary is the New Eve.
God created the first Eve immaculate (c.f. Gen. 2). After Adam and Eve sinned (c.f. Gen. 3), God says to the evil one, who took the form of a serpent:
[15] I will put enmities between thee and the woman, and thy seed and her seed: she shall crush thy head, and thou shalt lie in wait for her heel.
[15]
“She shall crush”: Ipsa, the woman; so divers of the fathers read this place, conformably to the Latin: others read it ipsum, viz., the seed. The sense is the same: for it is by her seed, Jesus Christ, that the woman crushes the serpent’s head.
If two parties are at enmity, they have nothing in common. Mary, the New Eve, had (and has) nothing in common with the devil since God preserved her from original sin at the moment of Her conception.
In 1854, Bl. Pius IX infallibly defined the dogma of the Immaculate Conception:
http://www.newadvent.org/library/docs_pi09id.htm
In 1858, the Most Holy Virgin appeared to St. Bernadette and said: “I am the Immaculate Conception.”
My story: 15+ years ago, I was invited to an Orthodox Baptism/Chrismation/First Eucharist by an Orthodox family I knew from work. They were customers at our store, and I frequently helped them so I went.
This is going to sound crazy but it’s the only way I can explain it: When I was in the narthex of the church (that’s where the rite of baptism begins in Byzantine practice), I felt a spiritual coldness which I could not describe. I was surrounded by people and physically warm (there was no air conditioning because it was in March). I did not participate in the liturgy. After the service, I talked to one of the deacons and asked him: Since your liturgy is the same as ours, and you profess that Mary is immaculate, why don’t you believe in the Immaculate Conception? He couldn’t give me an answer.
Sorry for the length of this post.